Mass seed production of mud crab Scylla serrata at Rajiv Gandhi Centre for Aquaculture

Mass seed production of mud crab Scylla serrata adopting the best management practices has been done in the hatchery of Rajiv Gandhi Centre for Aquaculture, Thoduvai in Tamil Nadu. Mature female mud crabs from farms and landing centres were procured, screened for White Spot Syndrome Virus and conditioned in the hatchery. The crabs were subjected to unilateral eyestalk ablation whenever necessary and maintained in tanks with sand substrate and aeration. The crabs were fed fresh squid, oyster meat and low value fish. Hatching occurred 9 days after spawning. The newly hatched zoeae were stocked in 5-ton tanks at 80 ind/li. The larvae were fed rotifers and Artemia nauplii. Green water culture system with the use of probiotics was adopted. Enrichment of Artemia was carried out. A hatchery run ranged from 25-30 days. The highest survival rate achieved was 17.5%. This could propel the commercialization of mud crab seed production in India

Status of mud crab industry in India

Mud crab is an economically important portunid crab species found in estuaries, coastal lagoons and nearshore waters of India. The genetics team of RGCA recently made an attempt to decipher the taxonomic ambiguity of mud crab species commonly available in Indian coastal waters using multiple molecular genetic markers and concluded that only two species of mud crabs, Scylla serrata and S. olivacea are commonly available in India. India has about 8,103 km of coastline with shallow coastal waters, brackishwater lakes, estuaries and intertidal swampy areas along the east and west coasts. It has been estimated that the potential resources of crabs particularly from the 7,770 km2 of estuaries and backwaters is 13,209 tonnes out of the total potential resources of 43,816 tonnes from Indian coastal waters. The southern part of the coast is potentially richer than the northern part. Estimated brackish water area in India is about 1,190,900 ha, out of which 167,193 ha developed for shrimp farming may become suitable for crab farming as evidently seen in Tuticorin, Tamil Nadu. Estimated mud crab culture area in Andhra Pradesh is more than 2,833 ha covering nearly 50 villages. Live mud crab export from India was higher in 2011-2012 (3,054 mt) compared to 2010-2011 (2,963 mt). Frozen and other forms of crab export declined in 2011-2012 (2,848 mt) compared to 2010- 2011 (3,251 mt). Apart from shrimp culture, mud crab farming, fattening and soft-shell crab production are now emerging as feasible business ventures in India. About 11 types of crab products are being exported from India with an average unit value realization of US$ 3.73 kg−1, highlighting its importance in the foreign exchange earnings. There has been a noticeable decline in the mud crab populations in the natural habitat throughout Indian coastal waters due to over exploitation and indiscriminate fishing of juvenile crabs by artisanal fishermen. Wild seeds are collected throughout the year in backwater zones of Sundarbans, Kakdwip and Namkhana of West Bengal; Chilika Lake of Odisha; coastal waters of Kakinada, Visakhapatnam and Rajahmundry of Andhra Pradesh; Pulicat Lake, Killai backwaters, Muthupet saline swamps, Punnakayal estuarine complex and Colachel coastal waters of Tamil Nadu; Neendakara, Cochin and Kozhikode backwaters of Kerala to meet the demand of culture operations. There is an urgent need to produce good quality hatchery seed to meet the demand of farmers. To address this, the Rajiv Gandhi Centre for Aquaculture (RGCA) established a state-ofthe- art mud crab hatchery for commercial scale production of seedstock to promote the adoption of the technology by the private entrepreneurs in the country

Molecular genetic approaches to resolve taxonomical ambiguity of mud crab species (Genus Scylla) in Indian waters

Mud crab or mangrove crab (Genus Scylla de Haan) is one of the most extensively cultured and economically important brachyuran crabs from the family Portunidae in Indo-Pacific countries, including India. Mud crabs exhibit variations in colour, size, spination, polygonal pattern and habitat. These contribute to the confusion in their identification. Accurate identification of the species is essential in the breeding programmes of domesticated stocks and is a crucial factor in the success of stock enhancement programmes. The taxonomic uncertainty of the genus Scylla in India is still an issue and several papers are being published using misleading identification. This is the first attempt to resolve the taxonomical ambiguity of mud crabs commonly found in Indian waters using multiple molecular genetic approaches. ITS-1, RAPD, PCRRFLP and mt-DNA sequencing along with traditional morphometric methods were used. Furthermore, a PCR method was developed by which mud crab species in India could be identified rapidly and accurately. The results of gene sequencing along with other molecular markers clearly indicated that the ‘green’ morph of Indian mud crab is S. serrata, while the ‘brown’ one is S. olivacea which was validated using the revised classification of mud crabs by Keenan et al. (1998). The S. serrata commonly mentioned in published literatures from India is S. olivacea and the S. tranquebarica, as believed by many Indian researchers, should be classified as S. serrata. Therefore, caution should be taken while interpreting or implementing the biological, molecular and aquaculture data published in those literatures

Soft-shell crab production using hatchery-produced mangrove crab Scylla serrata juveniles

Soft-shell crab production is being practiced in many Asian countries but the major source of seedstock is from the wild, which could no longer sustain the increasing demand. Commercial scale soft-shell crab production can become sustainable only if there is a continuous supply of seedstock not dependent on wild stock. Rajiv Gandhi Centre for Aquaculture (RGCA) under the Marine Products Export Development Authority (MPEDA) has established a pilot-scale mud crab hatchery at Thoduvai, Nagapattinam District, Tamilnadu in 2004 with the technical assistance from SEAFDEC/AQD. At present, RGCA has the biggest mud crab hatchery in India and a demonstration farm at Karaikal, U.T. of Puducherry, where nursery rearing, grow-out and soft-shell crab production are conducted using hatchery- produced mangrove crabs or mud crabs, Scylla serrata. This paper presents the results of the soft-shell crab production using hatcheryproduced S. serrata juveniles maintained in perforated low density polyethylene (LDPE) boxes. The duration of the molt interval, time of molt (day or night) and the increase in body weight and carapace width of each size group (40-60, 61-80, 81-100, 101-120, 121-140, 141-160, 161-180 g) after molting were determined. Results showed that the percentage increase in body weight and carapace width increased as the crabs grew bigger. Likewise, the molt interval was longer in bigger-sized groups of crabs. Sixty to seventy percent of the population molted during night time and the next molt occurred within 25 days in 80-160 g size groups. Soft-shell crabs in boxes within the water surface and juvenile crabs (2.5 cm CW initial size) for culture until market size in the pond can be a viable technology technology for mud crab growers